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EMERGING TECHNOLOGIES IN HAZARDOUSWASTE MANAGEMENT 8

An Overview

D. William Tedder1 and Frederick G. Pohland2

1School of Chemical EngineeringGeorgia Institute of Technology Atlanta, Georgia 30332-0100

2Department of Civil and Environmental EngineeringUniversity of Pittsburgh Pittsburgh, Pennsylvania 15261-2294

Several long-term trends in technology evolution have become apparent since these symposia first began in 1989. Earlier presenters more frequently discussed treatment methods involving extensive human intervention. Examples of this include soil inciner-ation (e.g., to destroy dioxin at Times Beach) and soil washing techniques that virtually reduce soil to sand and deplete most of its organic and inorganic nutrients. With suchharsh treatments, the residues are essentially beach sands with substantially less value, compared to their in situ uses prior to contamination. Moreover, the soil incinerationoption has inherent risks of airborne emissions.1

As the symposia have continued, the number of presentations describing extremelyharsh and expensive treatment technologies have gradually been supplanted by more subtle and gentler methods. Subsurface engineered barriers, for example, are often very helpful in controlling species migration beyond a designated area, but with lessobvious intrusion. Similarly, phytoremediation, the use of existing plant life or mixed cul-tures of microorganisms, to control hazardous species is continuing to emerge as an important technology. Bioremediation methods, particularly those utilizing in situ bacte-ria, are also of continuing interest as such species can be enlisted to control hazardous chemicals in many instances simply by providing missing nutrients (eg, phosphate or nitrogen).

The use of plants and bacteria to control organic species is readily obvious. Their use in controlling the movement and concentration of heavy metals is perhaps less obvious, but nonetheless fortuitous. Biological systems frequently exhibit high concen-tration factors for species, so trickle bed filters or other technologies utilizing suitable

Emerging Technologies in Hazardous Waste Management 8, edited by Tedder and PohlandKluwer Academic / Plenum Publishers, New York, 2000. 1

2 D. W. Tedder and F.G. Pohland

bacteria are finding increasing numbers of applications, and are often highly cost-effective. In exchange for meeting the requirements of sustaining bacterial life, mankind is able to exploit bacterial properties to control hazards, make the environment cleaner and safer and, by the way, also make it more attractive. Admittedly aesthetic perceptions are highly subjective, but it is suggested that marshlands, even when artificially created, are more attractive than settling ponds or other more obviously man-made devices.

Biological systems and engineered barriers may not be the final answers, particu-larly when heavy metals are concentrated in them, as these substrates must also be managed. Attractive methods must eventually be used for either recycling such metals to appropriate applications or placing them in appropriate final waste forms for permanent disposal. The latter method continues to be the favored approach for managing many radionuclides. However, biological systems can be a useful first step in concentrating toxicants or in some cases destroying them altogether.

Today, reasonable technical alternatives are available for managing many wastes, but questions still remain. Operational safety and user costs are paramount concerns, but better solutions are being found than those available only 10 years ago. The hazardous waste and environmental management problems resulting from industrialization at the turn of the century are not solved, but substantial progress has been made.

SOIL TREATMENT

In Chapter 2, Hong et al. address the use of chelating agents for extracting heavymetals from soil. Chelation is a potentially important method for removing metals (e.g.,Pb, Cu, Cd, Zn, Ni and Hg) from contaminated soil. This paper focuses on assessmenttechniques using predictions that are compared to experimental extraction and recovery results. The prediction of chelator complexing power, selectivity, and eventual chelator recoverability are of particular concern. Assessment techniques are outlined and results are presented for selected species from among 250 alternative chelators.

The ongoing development2–7 of electrokinetic soil remediation continues to suggestthe merit of this technology. In Chapter 3, Inman et al. examine the use of novelelectrodes and modulated reverse electric fields as a means of dealing with speciation issues that reduce overall efficiency. Modulated reverse electric fields, in conjunction with integrated ion exchange electrodes, are being studied as a means of inducing a more uniform remediation of contaminated soils, and to eliminate non-uniform pH profilesthat result from conventional electrokinetics and adversely affect speciation. Their process modifications control soil pH and enhance mass transport of heavy metals between electrodes.

In a related treatment, Rabbi et al. discuss the use of electrokinetic injection toenhance in situ bioremediation in Chapter 4 (also see Chapter 7). Their objective is touse electrokinetics to maintain higher nutrient concentrations in contaminated soil and thus to promote biological growth and destruction of the contaminants. They demon-strate the potential and versatility for this method to improve treatment at sites whereorganic contamination cannot be remediated by natural attenuation alone.

The National Research Council of Canada has developed a process for treating highly saline industrial soils contaminated with heavy oils and heavy metals found in Alberta, Canada. Majid and Sparks describe a solvent extraction soil remediation process in Chapter 5 . Heavy metal fixation is achieved by incorporating metal binding agents intothe soil agglomerates which form during the solvent extraction of organic species. After

Emerging Technologies in Hazardous Waste Management 8 3

remediation, the soils remain saline, but soluble salts can then be removed by water per-colation through a fixed bed of dried, agglomerated soil. Their goal is to provide an inte-grated treatment method which will adequately decontaminate soils, but still permitsubsequent agricultural use. More aggressive decontamination methods often render thetreated soil residues less effective for agricultural applications as mentioned above.

In Chapter 6, Martal et al. describe laboratory and field soil-washing experimentswith surfactant solutions, and summarize non-aqueous phase liquid (NAPL) recoverymechanisms. They studied solutions consisting of mixtures of anionic surfactants andalcohols, and other solvents in some cases. Their laboratory and field experiments show that residual NAPL recovery can exceed 90% while using less than six pore volumes ofconcentrated wash solution. Recovery mechanisms depend upon: (1) NAPL type, (2)micellar solution type (alcohol/surfactant or alcohol/surfactant/solvent), (3) ingredientconcentrations in the micellar solution, (4) washing direction (upward, downward, orhorizontal), (5) injection and pumping patterns, and (6) injection velocities.

In Chapter 7, Maillacheruvu and Alshawabkeh describe experiments to investigatethe effects of an electric field on anaerobic microbial activity, a topic related to thatdiscussed by Rabbi et al. in Chapter 4. Here the focus is less on the enhancement of nutri-ent transport properties by the electric field, but on effect of the electric field on micro-bial activity. They find that microbial activity, measured as a function of the ability ofanaerobic microorganisms to consume readily degradable acetate, generally decreases ifthe pH and dissolved oxygen concentrations are not controlled. Microbial activity ini-tially decreases under exposure to electric current, but recovers after a period of severalhours of exposure. This result suggests that to some extent the culture was able to accli-mate to the current, an important issue if an electric field is used to enhance the trans-port of biological nutrients through contaminated soils as suggested by Rabbi et al. inChapter 4.

GROUNDWATER TREATMENT

Results from studies on Fenton chemistry are reported in Chapter 8 by Tarr andLindsey. They focus on those chemical mechanisms which affect Fenton oxidation innatural waters. This investigation is an expansion on iron chemistry studies described inearlier volumes of this series.8–14 Their studies are related to others in this volume (Monsefet al. in Chapter 12, Greenberg et al. in Chapter 13, and Bower et al. in Chapter 14). Tarrand Lindsey find that altered hydroxyl production rates and increased hydroxyl scaveng-ing occur in natural waters. Hydroxyl binding to natural organic matter is also signifi-cant. They conclude that these three factors are key issues in determining the extent ofoxidation during pollutant destruction and reaction efficiency.They suggest that the same phenomena may also be important in soil systems.

Javert and Strathmann describe the use of surfactant and cosolvent to enhance the removal of non-aqueous phase contaminants (NAPLs) by modified pump-and-treatmethods in Chapter 9. Low NAPL recoveries may result from slow dissolutioninto groundwater, slow diffusion or desorption, or hydrodynamic isolation. They sum-marize ongoing surfactant and cosolvent studies that are designed to overcome these dif-ficulties and make pump-and-treat technology more generally applicable. This chapter complements earlier studieson NAPLs..15,16

Phytoremediation of TNT-contaminated groundwater by a poplar hybridis described by Thompson et al. in Chapter 10. This paper describes a pilot-scale

4 D. W. Tedder and F. G. Pohland

green-house experiment that examined the irrigation of a hybrid poplar tree (Populusdeltoides Xnigra) with TNT-contaminated groundwater. The results indicate that apoplar tree remediation system may be a feasible solution for low levels of groundwatercontamination.

In Chapter 11, Vidic and Pohland describe advances in treatment wall technology.This strategy involves the construction of permanent, semi-permanent or replaceableunderground walls across the flow path of a contaminant plume. The main perceivedadvantage for this technology is reduced operation and maintenance costs. Although con-siderable design details have already been developed through field- and pilot-scale appli-cations, some critical issues still remain to be resolved. These unresolved issues includeestablishing tested and proven design procedures, and evaluating interactions (perhaps favorable) with other groundwater remediation technologies.

In Chapter 12, Monsef et al. describe the removal of nitroaromatic compounds fromwater using zero-valent metal reduction and enzyme-based oxidative coupling reactions.Again, iron plays a key role. In this case they observe that zero-valent iron is effective inthe reduction of aqueous nitrobenzene to aniline in the absence of oxygen.

A modified Fenton oxidation process, based on a proprietary catalyst, is describedby Greenberg et al. in Chapter 13. In this case, a chelated iron complex was used toenhance the degradation of organic species at a contaminated site. Laboratory, pilot, andfull-scale experiments are described in studies aimed at destroying gasoline and waste oilconstitutents. After full-scale treatment, contaminant levels were either not detected, orwere reduced to concentrations below New Jersey groundwater standards. The contam-inated site was closed after one year.

Advanced oxidation process (AOPs), particularly those utilizing ozone, have beenof interest for a number of years. In Chapter 14, Bower et al. investigate techniques formaking AOP treatment less costly and more effective. They focus on the use of fixedbeds containing sands, especially sands with high iron and manganese concentrations. They find an enhancement in phenol degradation rates, possible due to the formation ofhigher concentrations of hydroxyl radical, at pH 7. Direct ozonation was equallyeffective at pH 8.9.

RADIOACTIVE WASTE TREATMENT

The development of final waste forms remains as an important topic of study, par-ticularly for radioactive wastes. Waste forms have been a recurrent theme in previousvolumes17–25 of this series. Chapters 15-17 of this volume continue this theme in which waste encapsulation studies utilizing polyethylene, by-product sulfur from refineryoperations, and polysiloxane are presented.

The investigation by Adams et al. in Chapter 15 considers the effectiveness ofusing polyethylene for encapsulating depleted uranium trioxide. Using a single-screwextrusion process, they were able to successfully process mixtures with waste loadings up to 90wt% UO3. Leach rates increased with waste loadings, but were relativelylow. Compressive strengths of samples were nearly constant in samples containing

In Chapter 16, Kalb et al. describe their initial test results investigating the use ofby-product sulfur from Kazakhstan to stabilize waste. The by-product sulfur is itself awaste. This stream is primarily elemental sulfur resulting from the refining of petroleumreserves in that country. Kazakhstan also produces hazardous and radioactive wastes;

50-90 wt% UO3.

Emerging Technologies in Hazardous Waste Management 8 5

using by-product sulfur to encapsulate other wastes could therefore “kill two birds withone stone.” Kalb et al. focus on the use of a sulfur-polymer cement21 by reacting the by-product sulfur with organic modifiers (5 wt%). This cement was then used to encapsulatewaste, achieving up to 40 wt% loadings. Waste form performance was characterized bymeasuring compressive strength, water immersion, and accelerated leach testing.

The encapsulation of nitrate salts using polysiloxane is described in Chapter 17.Durik and Miller present results in which compressive strength, metal leaching, and voidarea measurements of polysiloxane-encapsulatedwaste were measured as a function ofwaste loading (27–48wt%) for three surrogate wastes. Cost comparisons with immobi-lization using concrete suggest that polysiloxane could be an economical alternative for the U.S. Department of Energy which now manages substantial quantities of nitrate-bearing mixed wastes.

Regardless of the origin or nature of hazardous wastes, management invariably hasa common initial need to measure their properties. This can take many forms, but wasteassay is inevitable at some point and because the wastes are hazardous, operational risksoften result from such analyses. With radioactive wastes, nondestructive assay techniques,in which radwaste drums can be analyzed without actually opening them, have becomevery important. Such improvements in waste analysis26,27are always welcome.

In Chapter 18, Kottle et al. describe the development of an on-line analyzer for vanadous ion. This technique has potential applications in nuclear power plants in con-junction with the removal of corrosion products from heat transfer systems. Its use shouldreduce personnel exposures during such operations.

The use of magnetic separation for nuclear material detection and surveillanceis described by Worl et al. in Chapter 19. In this case, technology is being developed for the capture of submicron actinide particles, or the retrieval of fission products, in order to determine more about the operation of nuclear facilities. Using high-gradientmagnetic separation, they are able to separate paramagnetic compounds from those which are diamagnetic, and thus effect some degree of separation and concentrationof the paramagnetic species. Their work has potential environmental applications, particularly relating to environmental monitoring of actinide species at very lowcontamination levels.

Direct chemical oxidation28 of wastes is an alternative to incineration29–32 and elec-trochemical oxidation.33 In Chapter 20, Balazs et al. describe the application of transi-tion metal catalysts to enhance the ambient temperature destruction of organic wasteswith peroxydisulfate. This oxidation technology is potentially applicable to many solid or liquid organics (e.g., chlorosolvents, oils and greases, detergents, contaminated soilsand sludges, explosives, chemical and biological warfare agents). Silver ion has the great-est catalytic effect, but reaction rates are reduced somewhat by the presence of chlorideion. Catalysts also affect the production of chlorine gas when treating chlorinated organics, however, this effect is still not well understood.

SUMMARY

Emerging technologies in hazardous waste management are clearly diverse in natureand involve many different disciplines. This diversity is both an advantage and, poten-tially, a disadvantage. On the one hand, it offers the possibility of favorable collabora-tion between investigators with different backgrounds and emphases on the problem. Thisaspect can be advantageous. On the other hand, diversity can easily lead to some degree

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of confusion and misunderstanding. Thus it can also be a bit of a disadvantage, but suchdifficulties can be overcome if recognized and properly addressed.

Hazardous wastes still exist and although historical wastes have been reduced,major problems still remain, largely because of significant costs of remediation. Haz-ardous waste issues can nonetheless be solved if affected agencies, corporations, andpolitical entities are firmly and consistently resolved to do so. Clearly, technical progesshas been made since these symposia began in 1989. In many situations several technicalalternatives are available to deal with particular problems, but more progress is yet tobe made. Additional discovery is needed to further advance the basic science and tech-nology, and to make translation into practice more of a reality. While numerous techni-cal advances are being made, often their implementation is less than desired and neededto at last solve these problems. Thus, the real issue is less one of technology selection,but rather more one of social resolve and determination to see reasonable solutionsimplemented.

D. W. Tedder and F. G. Pohland

REFERENCES

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